Touch probes with movable armsets carrying feelers are used in coordinate measuring machines and in machine tools, in particular in machining centres and lathes, for checking pieces that have been or have to be machined, tools, machine tables, etc. In each one of such probes, the contact between the feeler and, for example, a mechanical piece is signalled by suitable devices that detect certain movements of the movable armset with respect to a casing and control the readout of transducers associated to the slides of the machine, the transducers providing measuring values with respect to a reference position or origin.
A detection and signalling device of a probe, like the probe described in the patent No. U.S. Pat. No. 5,299,360, can include an electric circuit and at least one associated switch that is mechanically actuated in consequence of displacements occurred between the movable arm and the casing and causes the closure or, more frequently, the opening of the circuit.
Other probes with detection devices including electric circuits placed at support and positioning systems are known for example from the patent No. U.S. Pat. No. 4,153,998. Other types of touch probes can include detection devices of very different kind and arrangement, among them strain or piezoelectric transducers. Patents publications No. U.S. Pat. Nos. 3,945,124, 4,177,568, GB-A-2049198, U.S. Pat. Nos. 4,462,162 and 4,972,594 show probes of this type.
Among them, the patent No. U.S. Pat. No. 4,972,594 shows a probe with an armset that includes arms and feelers and is connected to a fixed frame in a movable way. In particular, the armset is coupled to an intermediate support at mechanical reference abutments. The probe includes two detection devices: a laminar shaped piezoelectric transducer that generates a signal right after one of the feelers has touched the mechanical part to be checked, and an electric circuit closed by contacts in the above mentioned mechanical reference abutments. To avoid false signalling by the piezoelectric transducer, which is particularly sensitive to vibrations or other noises and to thermal variations, the signal of such piezoelectric transducer is considered as indicative of the contact actually occurred between the feeler and the mechanical part only if such contact is confirmed within a certain delay by the opening of the electric circuit caused by the armset movement and the consequent separation between the mechanical abutments. This procedure for ensuring immunity to noises of different types may cause problems due to the delay necessarily introduced in the contact detection. Such problems even increase when it is not possible to completely control the closing rate—set by the machine cycles—between the probe ad the part to be checked.
The piezoelectric transducer is—as already mentioned—thermosensitive, i.e. sensitive to thermal variations, and when changes in temperature occur it can produce electric signals that may cause false contact signalling.